In recent years, demands for higher integration and higher density of electronic parts have elevated electric power consumption per chip. Thus, effective removal of the generated heat in order to suppress temperature elevation of electronic elements is a critical issue. In view of the foregoing, alumina, particularly corundum (α-alumina), exhibiting excellent thermal conductivity, has become a candidate filler for a heat-dissipation spacer; a substrate material on which insulating sealing materials for semiconductors and parts of semiconductor devices are mounted; etc., and modification of alumina has been effected in a variety of fields.
Among such corundum particles, JP-A HEI 5-294613 discloses non-hollow, spherical corundum particles having no fractures and a mean particle size of 35 μm or less, the particles being produced by adding aluminum hydroxide and optional, known agents serving as crystallization promoters in combination to a pulverized product of alumina such as electrofused alumina or sintered alumina, and firing the mixture.
However, the above publication provides no clear description about corundum particles, in terms of the particle size, shape, etc., produced by use of electrofused or sintered alumina having a mean particle size greater than 35 μm.
There has been also known a thermal spraying method in which alumina produced through the Bayer method is atomized into high-temperature plasma or oxygen-hydrogen flame so as to melt and quench, to thereby produce roundish particles. Although the thermal spraying method provides coarse alumina particles having a size greater than 35 μm, unit heat energy requirement is large, resulting in high costs. In addition, the thus-produced alumina, though predominantly containing α-alumina, includes by-products such as δ-alumina. Such an alumina product is not preferred, since the product fails to exhibit properties required for alumina; e.g., lower thermal conductivity.
Pulverized products of electrofused alumina or sintered alumina have been also known as corundum particles having a size greater than 35 μm. However, these corundum particles are of indefinite shape having sharp fractures and produce significant wear in a kneader, a mold, etc. during incorporation thereof into rubber/plastic. Thus, these corundum particles are not preferred from a standpoint of practical use.
Several methods for producing a rubber/plastic composition exhibiting high thermal conductivity have been known; e.g., a method in which a high-thermal-conductivity filler such as aluminum nitride, boron nitride, or silicon carbide is incorporated, and a method in which a filler is added in as large an amount as possible. However, the former method is economically disadvantageous, since incorporation of the high-thermal-conductivity filler involves considerably high costs as compared with incorporation of α-alumina. When the latter incorporation method is employed, thermal conductivity of the resultant compound (composition) can be increased by virtue of incorporation of the filler at high concentration. However, the compound exhibits poor plastic fluidity, causing difficulty in molding thereof. Thus, a limitation is imposed on the amount of filler.
There has been studied another method in which particle size distribution is widened by incorporating fillers having particle sizes falling within a variety of ranges for enhancing plastic fluidity of the compound and increasing the amount of filler. However, since self-aggregation force of a microparticle component increases with decreasing particle size, fluidity is deteriorated upon incorporation thereof into rubber/plastic, and the component forms aggregated particles in the resultant rubber/plastic composition, possibly lowering thermal conductivity. Thus, a limitation is also imposed on the particle size that allows use of microparticles. With regard to a coarse particle component, coarse particles desirably have a nearly round shape for attaining high fluidity. However, as disclosed in JP-A HEI 5-294613, particles having a mean particle size generally greater than 35 μm and no fractures are difficult to produce.
In view of the foregoing, the present inventors have carried out extensive studies, and an object of the present invention is to provide a filler made of alumina whose particles impart less grinding and polishing performance and which can be incorporated in a large amount, through improvement of a process for producing roundish corundum particles.